An axial flow gas turbine engine includes a compression section, a combustion section and a turbine section. A flow path for working medium gases extends axially through the sections of the engine. As the gases are flowed along the flow path, the gases are compressed in the compression section and burned with fuel in the combustion section to add energy to the gases. The hot, pressurized gases are expanded through the turbine section to produce useful work and thrust.
A rotor in the turbine section has a rotor assembly for extracting useful work from the hot, pressurized gases. The rotor assembly includes at least two stages of rotor disk-blade assemblies. The rotor has a rotor shaft which connects these stages to another rotor assembly in the compression section to provide work to the compression section for compressing the working medium gases. A stator extends axially through the engine to circumscribe the rotor and to support the rotor through a bearing which is generally disposed in a bearing compartment. The bearing compartment is located in an inner cavity of the engine.
As the working temperatures and pressures of the engine are increased in modern engines, it has become necessary to provide pressurized cooling air to those rotor stages in the turbine that are closest to the discharge region of the combustion section.
A convenient flow path for providing cooling air to the second rotor assembly lies through the inner cavity containing the bearing compartment. Cooling air at a pressure high enough for use in the turbine section is ducted from a rear stage of the compressor. The temperature of the air is low in comparison to the turbine section, which is good for cooling, but high in comparison to the interior of the bearing compartment. Because of the high pressure needed for the turbine, this "cooling air" has caused problems in the bearing compartment. The hot air forces its way past the seals in the bearing compartment. The leakage of the hot pressurized gases past the seals into the bearing compartment is often accompanied by severe thermal distress resulting from small pockets of autoignition adjacent the seal region. As it turns out, operating bearing compartments avoided the problem of severe thermal distress by venting the bearing compartment to a very low pressure. Venting is no longer feasible because of the inability of known sealing means in the bearing compartment to maintain their integrity in the face of a large pressure differentials which must occur in current engines to supply cooling air to the turbine section at the correct pressure. Cooling all of the air flowed through the inner air cavity would solve this problem providing the pressure was maintained at suitable levels; but this is not feasible because of the large penalty in performance that accompanies the cooling of this air both in terms of parasite power and unavailable energy.
Accordingly, scientists and engineers are searching for methods of supplying hot pressurized air to a rotor assembly through the inner cavity in a way and with structure that prevents severe thermal distress in adjacent bearing compartments.